1. Field of the Invention
This invention relates to channel diffusers for supersonic centrifugal compressors and more particularly to curved channel diffusers which produce shock waves near the inner circumference thereof and which have channels that diverge at an increasing rate with increasing distance therealong from the inner circumference.
2. Description of the Prior Art
Single shaft gas turbine engines employ an impeller wheel coupled to rotate with a turbine wheel at a high rate of speed. Working gases enter the impeller wheel from a low pressure source such as the atmosphere and are expelled radially outward from the impeller with a high velocity which may have both tangential and radial components. A diffuser disposed about the periphery of the impeller receives the high velocity gases and converts the kinetic energy of the gases to static pressure. Economy and efficiency often require that pressure range of 6:1 to 10:1 be provided by a single stage compressor.
The efficiency of a gas turbine engine is strongly dependent upon the temperature and pressure of gases leaving the diffuser. Thus, even small changes in diffuser efficiencies which result in converting more of the kinetic energy to static pressure and less to heat can have important effects on engine performance. This is particularly true in engines having heat recuperators where the temperature decrease can be partly compensated for by an increase in useful heat transfer from the exhaust.
However, working fluid leaves the impeller wheel with very high velocities and complex flow patterns which make the design of optimal diffusers extremely difficult. If the flow path cross-sectional area increases too rapidly gas separates from the channel walls of a diffuser and extremely inefficient reverse flow develops along the walls. On the other hand, if the cross-sectional area increases too slowly the frictional losses along the channel walls are excessive. Further losses result from the inability of diffuser channels to anticipate the natural swirl of gases leaving the impeller wheel. Even more complexity is added by a transition from supersonic to subsonic velocities. The design of channel diffusers is thus a very complex and demanding art.
One channel diffuser arrangement is described in U.S. Pat. No. 3,333,762, "Diffuser For Centrifugal Compressor" to Vrana. Vrana describes a channel diffuser wherein each channel has a straight line longitudinal central axis near the inlet thereto which extends tangentially from the inner circumference. The channels are cylindrical near the inner circumference where adjacent channels intersect with one another and become conical about midway along the longitudinal axis to mate with a conical trumpet-like element near the outer circumference. Adjacent the inner circumference, intersecting adjacent channels are spaced so as to avoid the occurrence of a normal shock within the channels.
Another known channel diffuser arrangement teaches the use of a converging supersonic diffuser section followed by a diverging subsonic diffuser section. A shock wave is thus avoided. However, this arrangement is limited to a design operating speed where the MACH 1 transition occurs between the two diffuser sections. Furthermore, flow enters the subsonic diffuser section at a velocity of MACH 1. This high velocity creates large viscous and turbulent losses within the boundary layer and limits the efficiency of the diffuser. Effective pressure recovery diffusion is limited to an area ratio of about 5:1 and the high speed gases still possess a considerable amount of kinetic energy at the point where the 5:1 area ratio is exceeded. This energy cannot be recovered without an additional diffusion stage.